Enkephalinase or, more specifically, endopeptidase-24.11, is a mammalian ectoenzyme which is involved in the metabolic degradation of certain circulating regulatory peptides. This enzyme, which is a Zn.sup.+2 -metallopeptidase, exerts its effect by cleaving the extracellular peptides at the amino group of hydrophobic residues and thus inactivates the peptides as regulatory messengers.
Enkephalinase is involved in the metabolic degradation of a variety of circulating regulatory peptides including endorphins, such as .beta.-endorphin and the enkephalins, atrial natriuretic peptide (ANP), and other circulating regulatory peptides.
Endorphins are naturally-occurring polypeptides which bind to opiate receptors in various areas of the brain and thereby provide an analgesic effect by raising the pain threshold. Endorphins occur in various forms including .alpha.-endorphin, .beta.-endorphin, .gamma.-endorphin as well as the enkephalins. The enkephalins, i.e., Met-enkephalin and Leu-enkephalin, are pentapeptides which occur in nerve endings of brain tissue, spinal cord and the gastrointestinal tract. Like the other endorphins, the enkephalins provide an analgesic effect by binding to the opiate receptors in the brain. By inhibiting enkephalinase, the metabolic degradation of the naturally-occurring endorphins and enkephalins are inhibited, thereby providing a potent endorphin- or enkephalin-mediated analgesic effect. Inhibition of enkephalinase would therefore be useful in a patient suffering from acute or chronic pain. Inhibition of enkephalinase would also be useful in providing an antidepressant effect and in providing a reduction in severity of withdrawal symptoms associated with termination of opiate or morphine administration. In addition, inhibition of enkephalinase would also be useful in the treatment of irritable bowel syndrome.
ANP refers to a family of naturally-occurring peptides which are involved in the homeostatic regulation of blood pressure, as well as sodium and water levels. ANP have been found to vary in length from about 21 to about 126 amino acids with a common structural feature being one or more disulfide-looped sequences of 17 amino acids with various amino- and carboxy-terminal sequences attached to the cystine moiety. ANP have been found to bind to specific binding sites in various tissues including kidney, adrenal, aorta, and vascular smooth muscle with affinities ranging from about 50 pico-molar (pM) to about 500 nano-molar (nM) [Needleman, Hypertension 7, 469 (1985)]. In addition, it is believed that ANP binds to specific receptors in the brain and possibly serves as a neuromodulator as well as a conventional peripheral hormone.
The biological properties of ANP involve potent diuretic/natriuretic and vasodilatory/hypotensive effects as well as an inhibitory effect on renin and aldosterone secretion [deBold, Science 230, 767 (1985)]. By inhibiting enkephalinase, the metabolic degradation of the naturally-occurring ANP are inhibited, thereby providing a potent ANP-mediated diuretic, natriuretic, hypotensive, hypoaldosteronemic effects. Inhibition of enkephalinase would therefore be useful in a patient suffering from disease states characterized by abnormalities in fluid, electrolyte, blood pressure, intraocular pressure, renin, or aldosterone homeostasis, such as, but not limited to, hypertension, renal diseases, hyperaldosteronemia, cardiac hypertrophy, glaucoma and congestive heart failure.
In addition, the compounds of the present invention are inhibitors of Angiotensin-Converting Enzyme (ACE). ACE is a peptidyl dipeptidase which catalyzes the conversion of angiotensin I to angiotensin II. Angiotensin II is a vasoconstrictor which also stimulates aldosterone secretion by the adrenal cortex. Inhibition of ACE would therefore be useful in a patient suffering from disease states such as hypertension and congestive heart failure [See William W. Douglas, "Polypeptides--Angiotensin, Plasma Kinins, and Others", Chapter 27, in GOODMAN AND GILLMAN'S THE PHARMACOLOGICAL BASIS OF THERAPEUTICS, 7th edition, 1985, pp. 652-3, MacMillan Publishing Co., New York, New York]. In addition, it has been discovered that ACE inhibitors are useful in treating cognitive disorders [German Application No. 3901-291-A, published Aug. 3, 1989].
Bradykinin refers to a naturally-occurring peptide which is a very powerful vasodilator and causes increased capillary permeability. By inhibiting enkephalinase and ACE, the metabolic degradation of bradykinin is inhibited, thereby providing increased levels of bradykinin in the circulation.
In addition, the compounds of the present invention are useful as inhibitors of smooth cell proliferation. Smooth muscle cell proliferation in the intima of muscular arteries is a primary cause of vascular stenosis in arteriosclerosis, after vascular surgery, and after coronary angioplasy. Several animal studies have indicated the renin-angiotensin system plays an important role in this vascular response to injury. Chronic treatment with angiotensin converting enzyme (ACE) inhibitors reduced myointimal thickening following balloon injury in rat carotid artery or aorta. Powell, J. S., Muller, R. K. M. and Baumgartner, H. R.; Suppression of the vascular response to injury: The role of angiotensin-converting enzyme inhibitors. J. Am. Coll. Cardiol. 17:137B-42B, 1991. More recently, atrial natruiuretic peptide (ANP) has been found to decrease myointimal proliferation. ANP is rapidly metabolized by receptor mediated clearance and by neutral endopeptidase (NEP). Inhibition of NEP significantly reduces proliferation in the balloon-injured rabbit vasculature. Davis, H. R., McGregor, D. C., Hoos, L., Mullins, D. E. and Sybertz, E. J.: Atrial naturiuretic factor and the neutral endopeptidase inhibitor SCH42495 prevent myointimal proliferation after vascular injury. Circ. 86:I-220, 1992. These studies imply that a dual inhibitor of ACE and NEP should be therapeutically useful in the treatment of conditions which require inhibition of smooth cell proliferation. Davis and Sybertz, European Patent Application 533084-A1, Mar. 24, 1993.